Satellites are used for remote measurements of the earth in a wide variety of areas. These include remote measurements of the earth's atmosphere, its land surface, its oceans, measurements for defense, intelligence, and communications. In each field, various instruments are used to make active and passive measurements. For atmospheric measurements, examples are the remote profiling of temperature, the profiling of trace species, measurements of the wind field and measurements of the properties of clouds and aerosols. Thus, satellite remote sensing technology and methodologies may be used to perform a vast array of different measurements. In addition, satellite remote sensing has also been used for planetary measurements.
Measurements from low earth orbit satellites with altitudes in the range of 200 to 800 km provide measurements with high spatial resolution but with only relatively infrequent full earth coverage, for example, once per ½ day to a few days. On the other hand, measurements from satellites in a geosynchronous (GS) orbit provide frequent coverage, of the order of seconds to hours. Typically, a GS orbit is a very high altitude orbit at about 36,000 km altitude above the equator with the property that the satellite has a rotational period around the equator equal to the rotational period of the earth. Thus, a GS satellite stays positioned above the same point on the equator. This allows measurements from this platform to view a large portion of one side of the earth with frequent coverage.
The resolution of measurements from a satellite is limited by the effects of the earth's curvature, the scan angle of the instrument, and the altitude of the orbit. For a GS satellite, measurements made in areas at high latitudes have degraded resolution which for latitudes greater than 60° is significant. If the effective measurement area for a single satellite corresponds approximately to the range of latitudes from −60° to +60°, and, similarly, the range of angles in the equatorial plane from −60° to +60°, it follows that at least 3 GS satellites are required to cover the area in the plane around the equator. GS satellites do not, however, allow reasonable resolution to be obtained in areas above 60° latitude. Moreover, GS satellites do not allow any coverage in the Polar Regions, since a view of this area is blocked by the curvature of the earth and the approximate 8.6° angle of the poles as seen from a GS satellite. For 3 GS satellites, the net angle of measurements of the earth at the edge of the scan in the equatorial plane or at ±60° latitude is approximately 68°.
High altitude GS or stationary satellites in equatorial orbits at about 36,000 km altitude collect earth imaging and communication data for various different user communities. The data include the ultraviolet, visible, near infrared, infrared, microwave and radar frequency regions. The data collected are often in the form of digital imaging data which can be used to make pictures or imagery in one or potentially thousands of spectral regions for photographic or computer aided analysis. The data can be collected in the frame time, a short period of time of the order of seconds to hours, with about 3 satellites from about −60° S to +60° N latitude. Close to full earth coverage can be obtained with an additional 3 to 6 high altitude satellites. The data obtained are low resolution, however, even for very large expensive systems because of the very high altitude orbits.
LeCompte, in a series of six patents and patent applications (U.S. Patent Application Nos. US 2003/0095181A1, May 22, 2003; US 2002/0089588, Jul. 11, 2002; US 2002/0041328, Apr. 11, 2002; U.S. Pat. No. 6,271,877 issued Aug. 7, 2001, U.S. Pat. No. 6,331,870 issued Dec. 18, 2001; and U.S. Pat. No. 6,504,570, issued Jan. 7, 2003) discloses a system for measurements from a geostationary, geosynchronous orbit. He provides a system, methods, and apparatus for collecting and distributing real time, high resolution images of the earth with a sensor based on multi-megapixel CCD arrays. The system utilizes at least four, 3 axis stabilized satellites in geostationary orbit to provide world-wide coverage excluding the poles. The current disclosure uses non-geosynchronous orbits at various altitudes to provide measurements with much higher performance and resolution, full earth coverage including the polar regions and areas at high latitudes, and significant cost advantages over prior art systems.
The use of a zoom type feature, which is similar to the zoom feature of a camera, has been used on geosynchronous GS platforms. This feature allows a small area of the earth to be observed with more detail than would normally be the case. However, in order to obtain this detailed view, the coverage of the rest of the earth that would normally be viewed by a given satellite is lost. For a one satellite GS system, this would result in a loss of all of the data that would normally be obtained, and in a 3 satellite GS system this would result in a loss of ⅓ of the data, which severely limits the use of this feature.
For a low or mid-altitude satellite (LMAS) constellation, an ultra-high performance measurement can be made in any given small area of the earth by acquiring and viewing only that area when it first appears in a portion of the area being scanned by a given satellite. The coverage of the small area continues until the satellite leaves the coverage area and as coverage of the small area of interest from a given satellite ends, coverage of the small area of interest from the following satellite begins. This process provides continuous coverage of the small area of interest from successive satellites. For a system with n satellites, up to n ultra-high performance measurements of small areas can be made simultaneously, one measurement for each satellite.
The only loss of coverage as a result of the use of the ultra-high performance feature is in the single area containing the small feature of interest. For an LMAS system with 200 satellites, this would result in a loss of coverage of 1/200 or 0.5% of the coverage of the earth. Moreover, an LMAS system has a lower altitude than a GS system, which advantageously allows a much higher diffraction limited resolution than a GS system, e.g., more than 100 times higher resolution for a 200 km altitude LMAS system.
Luders (1961) described constellations for continuous, complete global coverage using computer search methods with the “streets of coverage” technique for polar and inclined orbit constellations. Rider (1985) and Adams and Rider (1987) described continuous global coverage for single, double, . . . k-fold redundant coverage using the “streets of coverage” technique for optimal, i.e., the minimum number of satellites for a given coverage, polar orbit constellations. They also described k-fold redundant coverage for latitudes above given latitudinal planes, i.e., 0°, 30°, 45°, and 60°. Rider (1986) used the “streets of coverage” technique to obtain constellations for inclined orbits. Walker (1970) used circular polar orbits and, in 1977, used inclined orbits to obtain orbit constellations using computer search and analytic techniques where each satellite of the constellation can have its own orbital plane. This work was used only for the case of very small satellite constellations.
The “streets of coverage” technique uses a conical scan pattern to scan the area within a series of minor circles which are centered at each sub-satellite point on the spherical surface of the earth. The area of continuous overlay of these circular patterns on the earth at a given point in time for satellites in one orbital plane and for satellites in different orbital planes then describes the area of continuous coverage. For continuous whole earth coverage, Rider gives a complex analytic method for determining the orbital planes, the number of satellites, the effective “streets of coverage” angle of the conical scan pattern, and the multiplicity of the redundant, i.e., single, double, or k-fold coverage as a function of these parameters. Results are given for optimally phased and unphased polar satellite constellations for constellation sizes up to approximately 160 satellites for single coverage.
Patent WO 03/040653A1, filed on Nov. 11, 2002 by A. B. Burns, “Improved Real or Near Real Time Earth Imaging Information System and Method for Providing Imaging Information” claims to provide methods for contiguous or overlapping coverage over the earth (95% of earth) for imaging measurements from relatively low altitude elliptical polar orbiting satellites at 640 km, as well as for other non-satellite platforms. On pages 52-55 of this patent, “the specific configuration of the satellite network having particular regard to how real time global coverage is achieved” (p. 52), is described. The method, calculations, and instructions the patent gives for obtaining contiguous, overlapping coverage “so that the footprints contiguously and concurrently cover a substantial part of the earth's surface continuously and dynamically” (p. 4), for 95% of the earth's surface are as follows. First, the surface area of the 95% of the earth covered is calculated as 4.856*108 km2 (p. 53, lines 11 and 12). The effective area covered by a single satellite is then calculated as 212,677.58 km2 (p. 54, line 21). “The number of satellites required to image a given proportion of the earth's surface” (p. 55, lines 1-2) is then given as
                                                                        Number                ⁢                                                                  ⁢                of                ⁢                                                                  ⁢                satellites                            =                            ⁢                              area                ⁢                                                                  ⁢                to                ⁢                                                                  ⁢                be                ⁢                                                                  ⁢                                  covered                  /                  area                                ⁢                                                                  ⁢                of                ⁢                                                                  ⁢                coverage                                                                                        =                            ⁢                              4.856                *                                                      10                    8                                    /                  212677.58                                                                                                        =                            ⁢                              2283                ⁢                                                                  ⁢                                  satellites                  .                                                                                        (        A        )            
Equation (A) requires that each satellite in the constellation of satellites cover a different area on the earth of the same size, 212,677 km2. That is, the satellites must provide uniform spatial coverage over the earth. FIG. 31b in the Burns patent shows a series of these contiguous coverage areas of equal size. However, Eq. (A) and the Burns patent do not give a correct method for how this uniform coverage could be achieved for the case of satellites.
For example, FIG. 28 is one of the four detailed figures for imaging in the Burns patent. It is described in the brief description of the drawings as “a diagram showing how a single polar orbiting satellite images the earth's surface”. It shows a series of parallel orbits going in a north-south direction which have approximately uniform width in the east-west direction and appear to give uniform coverage. These orbits, however, do not go over the poles (with the exception of the central orbit) and are thus not the required polar orbits as specified in the patent. These orbits also do not go around a circumference of the earth (with the exceptions of the central orbit), that is, they do not make great circles or ellipses around the earth, and therefore, are not possible satellite orbits. Thus, this figure does not describe satellite orbits except for the central orbit and cannot provide the uniform coverage specified in the patent.
The Burns patent also states that elliptical polar orbits are required and that circular polar orbits will not work. Burns, however, only attempts to treat the case of a circular polar orbit as described by Eq. (A) with an orbit altitude of 640 km and does not attempt to treat the case of an elliptical orbit.
Polar orbiting satellites have the property that the satellite orbit passes over the north and south poles. As shown in FIG. 1 of the Burns patent, and similar figures in other patents, constellations of polar orbits have maximum orbital separation at the equator, the orbits converge and the separation decreases at mid-latitudes, and the satellite orbits converge and essentially totally overlap as the orbits approach the poles. As a result, the satellite spacing is maximum at the equator in the longitudinal, east-west, direction, decreases significantly at mid-latitudes, and goes to zero at the poles. The corresponding coverage per satellite is highly non-uniform with the amount of coverage and overlap varying by more than 20 times over the surface of the earth for the case Burns considers.
As discussed above, Eq. (A) assumes uniform satellite coverage over the earth to calculate the number of satellites. Since this does not occur for polar orbiting satellites, Eq. (A) and the Burns method is fundamentally incorrect. Further, if Eq. (A) is used, then overlap in coverage in one area of the earth, e.g., as occurs in the longitudinal direction at high latitudes for polar orbits, must be compensated for by corresponding large gaps in coverage in other areas of the earth. These gaps in coverage result from the Burns methodology and do not allow the contiguous/overlapping claims of the Burns patent to be realized for satellites.
The equations in this disclosure can be applied to the Burns patent parameters to calculate the number of satellites required for full earth coverage. The chord length of his measurement 2X is determined from the square root of Burns' effective area of coverage, 212,677.6 km2 for a single satellite, which is a square, and which yields 2X=461.17 km. From Eqs. (1) and (2), the number of polar planes required as the satellites pass through the equatorial plane are calculated as ne=44, and the required number of satellites per polar plane from Eq. (4) as np=87. This, in turn, gives the minimum number of satellites needed for coverage over the earth from Eq. (6) as 3828. This calculation includes the effects of overlapping coverage.
The 3828 satellites required based upon the foregoing are considerably larger than the 2283 satellites determined by Burns using his Eq. (A). To get full earth coverage in the polar plane with the Burns parameters, 87 satellites per polar plane are required. When the total number of satellites calculated in the Burns patent is divided by 87 satellites per polar plane, we find that only 26.24 planes would be available across the equatorial plane. This would provide only approximately 60% coverage and would give 40% gaps in coverage in the equatorial plane, i.e., 26.24/44≈0.6. A similar result is obtained for the percent coverage and gaps in coverage in the polar plane.
Thus, it is clear from the preceding detailed calculation that to compensate for the overlapping coverage inherent in constellations of polar orbiting satellites, the Burns patent methodology produces large gaps (approximately 40%) in full earth coverage. Since his stated design was for 95% full earth coverage, the effective gap in coverage in the Burns methodology is about 35%. Thus the contiguous/overlapping claims of the Burns patent cannot be met for satellite application.